This invention concerns an improved system and method for synchronizing the horizontal sweep circuitry of a video display with a horizontal drive signal.
In general, video information is displayed by a television receiver on a raster which is scanned horizontally at a first rate, and scanned vertically at a second, generally slower rate. The video input information is presented as amplitude-modulated synchronizing pulses by which the raster scanning of the television receiver is synchronized with the information to be viewed. For proper picture framing, it is required that the frequency and phase of oscillation produced by the horizontal sweep system be completely synchronized with those of the horizontal synchronizing signal transmitted from the broadcast station. The synchronization requirements of television receivers are eased somewhat by the standardization of television waves in which the frequencies of the horizontal and vertical synchronizing signals substantially satisfy a predetermined relationship.
This standardization of synchronizing signal relationships has significantly reduced operating requirements of television receivers in this area. The most common approach to signal synchronization in television receivers utilizes a phase locked loop in which a phase detector has horizontal rate synchronizing pulses applied to one input and ramp signals related to the horizontal deflection applied to the other input. The phase detector responds to these inputs and produces control pulses which are filtered and used to control a horizontal-rate oscillator at a frequency which is an average of the incoming synchronizing signals. The horizontal oscillator in turn drives the horizontal deflection generator producing recurrent trace pulses which are integrated to produce the recurrent ramp or sawtooth signal input to the phase detector. Operating limitations in a phase locked loop system arise, however, when input synchronization signals vary over a large frequency range. This variation of input synchronization signal frequency becomes a critical operating parameter when a video display is used in a non-television type of application. For example, a video display used in a computer terminal or in a data display presentation system may be required to interface with a great variety of input synchronization signals. The typical television receiver not only lacks the flexibility to interface with this great variety of input signals but also is incapable of controlling the presentation to fit a particular performance requirement, e.g., using the periphery of the display's raster to present data of one type and the center of the raster to present data of another type.
Various approaches have been undertaken to make video displays more compatible with a greater variety of input synchronization signals. One such approach is described in U.S. Pat. No. 3,794,760. Described therein is a horizontal synchronizing device for a television receiver utilizing a mechanical resonator formed from piezoelectric ceramic material to drive an oscillator at a frequency equal to the input synchronizing signal. This results in the oscillation frequency being drawn toward the frequency of the input synchronizing signal so as to be in agreement therewith. With the oscillator thus synchronized, the disclosure emphasizes the high synchronization stability of this system due to the inherent stability of the oscillator itself. However, the disclosure fails to point out the inherent performance limitations in this system arising from the use of a mechanical resonator, the stability of which is strongly influenced by such factors as aging, temperature variations, and other operating and environmental factors.
Another approach to improving video display synchronization performance is disclosed in U.S. Pat. No. 3,487,167 which passes the composite horizontal and vertical synchronization pulses but separates out all video signals by means of an amplitude separator. Blanking out vertical sync pulses and gating through horizontal sync pulses to the horizontal sync output gate and blanking out horizontal sync pulses while gating through vertical sync pulses to the vertical sync output gate is accomplished by means of two one-shot multivibrators in series. The multivibrator originally is in a nonoscillating state with a trigger signal required to start the single cycle of operation. While this approach reduces jitter in horizontal synchronization at the beginning of the horizontal and vertical interlaced fields in each frame, it does not significantly increase the range of synchronization signals with which a given video display may operate.
Another approach to video display synchronization with incoming signals is disclosed in U.S. Pat. No. 3,430,067 which is directed toward application in a television receiver. This invention permits the production of the 60 Hz field frequency signal directly from the 15,750 Hz line frequency signal by dividing the latter by a non-integer factor. To divide by the non-integer, 262.5, the preferred embodiment includes a frequency divider that divides by 17 and then by 18 in response to a bi-stable multivibrator. The output of this frequency divider directs a monostable multivibrator and a differentiator to provide a pulse train having a series of equally-spaced pulses which have a frequency equal to the line frequency divided by 17.5. While this approach simplifies and improves synchronization in a television receiver, it is limited to a line frequency input of 15,750 Hz.
The present invention, however, utilizes a combined monostable and astable multivibrator configuration to provide video display synchronization with a great variety of input signals. In addition, the present invention, unlike the previously described approaches, is capable of converting a drive pulse or a synch pulse to a properly phased and timed drive pulse.